How do you change a question to multiple choice from single choice?

Our JotForm has a question that says "select all that apply", but the form element chosen was for a single choice question. Is there a way to change this even though the form has been published?

Yes, you just need to go into the edit mode of your form:

And replace the field. However, please note that if you had collected data from that single choice field already, once deleted its associated data will be deleted too.